The present invention relates to the reduction of electromagnetic interference (EMI) in a broad sense, including emissions, susceptibility and information leakage, and in particular to the reduction of unintentional electric current on shields of one-shielded and multi-shielded cables.
Electromagnetic interference in electronic equipment is generated by the presence of disturbing intentional or unintentional electromagnetic fields. Intentional electromagnetic fields are deliberately generated for some special purpose, for example fields generated by wireless communication antennas or by broadcast antennas. Since time varying currents generate electromagnetic fields, unintentional electromagnetic fields can be generated by any electronic equipment. Digital circuits are broadband sources of unintentional electromagnetic radiation, and they are often in contact with or in proximity of conductors of dimensions comparable with the wavelength at some particular frequency. Such conductors can act as effective source or receiving antennas.
Conducting cables are an example of effective radiating antennas, particularly due to discontinuities of the cross section along the cable length, such as bends or terminations. Cables can act also as unintentional electromagnetic waveguide and contribute to increase the conducted EMI, or provide the path between the source and the antenna. High frequency electromagnetic waves can be guided by single or multi-conductor cables. In the case of two conductors, two fundamental propagation modes are possible at any non-zero frequency, which are sometimes called differential mode (DM) and common mode (CM) depending on the current polarity 18, as shown in FIGS. 8 and 9.
In the case of more than two conductors the number of fundamental modes increases, as shown for example in the non patent document 1, where however the CM is not considered. For any number of conductors it is possible to define a propagation mode with the same polarity for the current in all the conductors, and this will be called CM or universal common mode (UCM) in the following. The total current carried by the UCM is the algebraic sum of all the currents flowing on all the conductors. The remaining fundamental modes will be called differential modes (DMs) in this patent.
In a similar way, electromagnetic waves can be guided on the external surface of the shield or shields of multi-shielded cables. If the shielding effectiveness is large enough (well shielded cables), the field inside and outside the cables can be considered uncoupled and in the UCM definition the internal conductors, including the internal part of the shields, should not be considered. For example, in the case of a single well shielded cable, like a coaxial cable, the current flowing outside the cable will be called CM or UCM current. The value of shielding effectiveness necessary for considering the field as uncoupled depends on the application of interest, and on the relative intensities of the field inside and outside the cable, for example a 1% reduction of the field would require approximately 40 dB of shielding effectiveness. Internally to the shield, the shield can be taken as reference conductor and a local definition of common mode (LCM) with return current on the internal part of the shield can be used for multi-conductor cables such as the twin axial (twinax) cable.
On the external part of the shields the DM currents are a sort of noise, and the term shield differential modes (SDMs) will be used, in order to distinguish them from the DMs selected for the intentional signal currents on other eventually present unshielded cables. The SDMs include modes made by a combination of currents on shields of shielded cables and on unshielded cables. For convenience the term shield common mode (SCMs) will be used for the UCM when only shielded cables are present, and for those SDMs having the same current polarity on all the shielded cables, but a different polarity in at least one unshielded cable.
In the case of cables with two or more shielding levels, such as the HDMI cable, or a combination of these cables eventually also in configurations with higher shielding levels, local definitions of the previously defined modes are possible for well shielded cables. This means that the previous definitions can be applied to the highest shielding level outside all the shields, as well as inside intermediate shielding levels, with local SDMs and SCMs.
The present invention relates with the reduction of the unintentional currents flowing on cable shields, that is the SCM current and the SDM currents. In the embodiments of FIGS. 3 and 6 only the most external shields are considered. In the embodiment of FIG. 7 the shields of any shielding level are considered.
The mentioned problems related to EMI are usually indicated as conducted and radiated emission problems. Obviously for the same reasons, current on cable shields plays a role also in the reciprocal problem of the susceptibility of electronic equipment to EMI, to the related problems of immunity by electrostatic discharge, EMP (electromagnetic pulse) and lightning. Another important issue is that of prevention of information leakage through direct or indirect coupling with shielded cables present in the environment, not necessarily attached to the emitting electronic equipment. By aiming at reducing the current flowing on the cable shields, the present invention relates at least with all these fields.
In order to reduce the shield current EMI filters are often used, in many cases based on a similar principle to those used for CM current for example in the foreign patent document 1: U.S. Pat. No. 4,506,235. Ferrite chokes have been proposed in many patents, such as the foreign patent document 2: U.S. Pat. No. 6,867,362 and the foreign patent document 3: U.S. Pat. No. 6,335,483. A combination of ferrite chokes at different frequencies has been proposed for example in the foreign patent document 4: U.S. Pat. No. 5,287,074. Sheets of absorbing materials have been proposed for example in the foreign patent document 5: U.S. Pat. No. 5,990,417.
One advantage of this type of filters is that they can be applied to both shielded and unshielded cables. Usually they do not require an electrical connection to the cable and can be applied without any particular effort to a manufactured cable. They are also wide-band filters. On the other hand, the maximum frequency is limited by the magnetic properties of the used materials, typically below 1 GHz. The reduction of CM current is not always sufficient, typically below 10 dB. Even though no experimental evidence is available, the reduction of SDM current in multi-shielded cables is likely to be even smaller than the reduction of the SCM current, due to the different spatial distribution of the electromagnetic field on the cross section of the cable, because for the SDM currents the energy is mainly concentrated in the space between the shields.
If a filter requires an external connection to the cable shield, such as a grounding connection, the external insulator must be removed. Internal connections between shield and an internal conductor by means of openings through the insulator have been considered for example in the foreign patent document 6: U.S. Pat. No. 3,469,016. External connections to a cable shield are shown for example in the foreign patent document 7: U.S. Pat. No. 4,257,658. This type of connections are typically used for grounding the cable, as explicitly mentioned for example in the foreign patent document 8: U.S. Pat. No. 5,597,314. Connections of a bundle of shielded cables for grounding have been considered for example in the foreign patent document 9: U.S. Pat. No. 6,485,335.
This type of connection is aimed to relatively large and resistant coaxial cables. Grounding of a bundle of micro-coaxial cables has been proposed in the foreign patent document 10: U.S. Pat. No. 6,413,103 B1. In the latter patent, originally separated coaxial cables are electrically contacted together in the transversal direction in one or more position along the cable length by means of two conducting plates a the top and at the bottom, after the removal of the external insulator (jacket) of the coaxial cables. The purpose of the foreign patent document 10 is to reduce EMI by means of one or a plurality of connections to ground, represented by a larger conductor such as the chassis of a portable computer. In general grounding is effective when the distance between the cable and the grounding surface is much smaller than the wavelength.
High frequency serial interface cables have nowadays a clock frequency above 1 GHz. This results in strong EMI emission peaks at few frequencies in the GHz region, related to the interface clock frequency. These large and relatively isolated noise frequencies can be more effectively reduced with narrow-band filters, or with a combination of narrow- and wide-band filters. One disadvantage of narrow-band filters is that they require a more accurate design in order to tune them to the correct frequency. A second disadvantage is that in order to optimize the filter, information on the noise spectrum is necessary. Therefore, depending on the circumstances, the filter cannot usually be designed together with the cable, except for example for cables manufactured for a known specific interface clock frequency.
One of these filters is a quarter-wavelength CM suppressor sleeve, discussed for example in the non patent document 2. This type of filters is an extension of the well known sleeve or bazooka balun, discussed for example in the non patent document 3. The fundamental idea is that a quarter-wavelength long waveguide that is shorted at one termination appears ideally as an open circuit at the opposite terminals. One difficulty in designing these sleeve CM chokes is that in practice the frequency at which the resonance minimizing the CM current transmission occurs, does not correspond exactly to the frequency at which the sleeve is one quarter-wavelength long. The optimal sleeve length depends in practice also on the diameter of the sleeve.
Baluns are used at the transition between balanced and unbalanced cables with the purpose of improving the transmission between the respective propagation modes. The reduction of the CM current is a useful and necessary effect, even though it is not the main purpose. On the other hand the purpose of the quarter-wavelength sleeve choke is to reduce the CM current on a single cable, and therefore some differences in the implementation may exist. For example, the source of CM current does not need to be the transition between a balanced and an unbalanced line. Furthermore, the position of the sleeve does not need to be exactly at the terminals, even though a proximity to the CM source is preferred. For this reason it is also possible to cascade sleeve chokes at difference frequencies, as an alternative to using more complex extensions such as dual frequency sleeve baluns, which are discussed for example in the non patent document 4 and can be applied also to sleeve chokes.
One patent related to the quarter-wavelength sleeve CM suppressors is the foreign patent document 11: U.S. Pat. No. 6,284,971. The field of the invention is slightly different, since the invention is a cable for magnetic resonant imaging. In that case the cable is carrying a single frequency and large power signal, and the CM current can increase the temperature of the cable creating safety risk for the patient. Applications to other fields such as EMI with antennas are considered in the patents. The patent includes a cable having a plurality of sleeve chokes of length corresponding to around one quarter of the wavelength at the operating frequency.